WO2019080969A1 - EXTRUDERS, EXTRUDER AND METHOD FOR PRODUCING AN EXTRUDER NUCLEAR, AS WELL AS PLASTIC FORMING APPARATUS AND METHOD - Google Patents

Abstract

Extrusion screws are subjected to wear phenomena related to corrosion and / or abrasion, in particular when processing plastics which have a high filler content and / or flame retardant and / or require high processing temperatures. The present invention relates to a wear protection for an extrusion screw, obtained by a surface treatment of the extrusion screw and a tempering process and simultaneous or subsequent income of said extruder screw.

Description

 EXTRUDERS, EXTRUDER AND METHOD FOR PRODUCING AN EXTRUDER NUCLEAR, AS WELL AS PLASTIC FORMING APPARATUS AND METHOD

The invention relates to an extruder screw, an extruder and a method for producing an extruder screw, as well as a plastic-forming system and such a method.

In particular, the invention relates to an extruder screw with a wear protection, in particular against corrosion-induced and / or abrasive wear, in particular wear, which occurs in the processing of plastics which have a high filler content and / or a flame retardant and / or require high processing temperatures Extruder with an extruder screw having such wear protection and a method for producing such a wear-protected extruder screw.

A well-known field of application of extruder screws include injection molding machines, which produce a melt from a starting material in an extruder discontinuously alternating between a metering and an injection process and injects them through a nozzle into a tool, whereby an item is formed as a product in the tool.

Another known field of use of extruder screws are extrusion machines which continuously produce a melt from a starting material in an extruder and discharge it through a shaping opening. Endless products are produced in extrusion machines.

Generally speaking, extruder screws are used for the plasticization of plastic materials. In this case, extruder screws are subject to corrosion-induced and / or abrasive wear, in particular in the processing of plastics which have a high filler content and / or a flame retardant and / or require high processing temperatures.

This wear can lead to premature failure of an extruder screw, resulting in a maintenance interval and maintenance costs. In the prior art, it is known to through-cure extruder screws by the use of a heat treatment process, so as to increase the wear resistance of the extruder screw.

Powder-metallurgical steels are used especially for special requirements on abrasive and corrosive resistance. These contain commercially available up to about 20 °% chromium as an alloying ingredient. Chromium forms chromium carbides with carbon, which protect against abrasive wear as hard materials.

Depending on the heat treatment process, the effect of chromium, either as a hard material against abrasive wear or as a passive layer against corrosive attack, can be more pronounced. Under a passive layer is understood to mean free chromium on the surface, which is particularly suitable to improve the corrosion resistance of the extruder screw.

Furthermore, known in the art so-called armored extruder screws are known.

In cross-cut extruder screws, the extruder screw is first formed from a core material into a first shape and then a different material from the core material is applied by cladding predominantly on the surface of the extruder screw webs, so that in the extruder screw webs a composite material is produced which, depending on the selection of the different materials, should ensure the widest possible protection against abrasion and corrosion.

In practice, extruder screws with an extruder screw diameter of up to approx. 50 mm are predominantly through-hardened, whereas extruder screws with a larger diameter of the extruder screw have hitherto been mainly bar-reinforced.

The invention has for its object to provide the prior art an improvement or an alternative.

According to a first aspect of the invention, the object solves a process for producing an extruder screw, for use in an extruder for conveying, melting and homogenizing a starting material, in particular a starting material comprising a polymer, comprising the steps of (i) preparing an extruder screw precursor from an extruder screw precursor alloy; and (ii) performing a thermochemical surface treatment to differentiate the extruder screw precursor to an extruder screw core and a surface rim zone, wherein the extruder screw precursor is exposed to a medium, the medium being different from a regular earth atmosphere by an increased chromium and / or boron content. Conceptually, the following is explained:

First of all, it should be expressly pointed out that, in the context of the present patent application, indefinite articles and numbers such as "a", "two" etc. should as a rule be understood as "at least" information, ie as "at least one ...", "At least two ...", etc., unless it is expressly stated in the context in question or it is obvious or technically compelling to the skilled person that only "exactly one ...", "exactly two ..." etc. may be meant. An "extruder" is a machine that retracts, feeds, melts, homogenizes and continuously extrudes a raw material from a shaping orifice or dispenses it into a tool in a dis- proportioned manner An extruder may have one, two or more than two extruder screws.

An "extruder screw" is a particularly important machine element of an extruder.Together with the cylinder, it forms the plasticizing unit, in which the plastic is melted.The geometry of the screw determines the delivery uniformity, quality and delivery rate.The extruder screw has the task of the art - To feed the material into the cylinder, to convey it, to melt it and to homogenize it, as well as to build up a pressure to flow through the tool.

A "starting material" refers to all the starting materials used in the broadest sense in the plastification of, in particular, thermosetting and / or thermosetting polymers, including additives and marker materials, etc. The starting material usually has a special formulation Glass fibers.

A "polymer" is a chemical composed of macromolecules, and in many cases a polymer consists of non-identical macromolecules Synthetic or semisynthetic polymers are the main component for the production of plastics An "extruder screw core" is understood to mean the core material of an extruder screw. The surface edge zone is a surface edge layer of the extruder screw, which can be a few microns up to a few tenths of a millimeter or even a few millimeters thick The thickness of the surface rim zone is determined by the surface treatment method Accordingly, the surface treatment process differentiates between the surface skin layer and the extruder screw core. By an "extruder screw precursor" is meant an untreated extruder screw billet. Through the surface treatment, the surface treatment process differentiates the extruder screw precursor into the surface skin layer and the extruder screw core.

By an "alloy" is meant a metallic material consisting of at least two elements which together have the metal-typical characteristic of the crystalline structure with metal bonding.

The behavior of the elements in an alloy and their influence on their properties are generally dependent on three factors: type and number of alloying partners, their mass fraction of the alloy and the temperature. These factors determine the respective receptivity, ie solubility of one element in the other and whether the alloying partners form mixed crystals or mixtures of pure crystals of the respective alloy components.

By an "extruder screw precursor alloy" is meant the alloy of the extruder screw precursor.

An "extruder screw core alloy" is understood to mean the alloy of the extruder screw core.

By a "surface edge zone alloy" is meant the alloy of the surface edge zone.

A "surface treatment" is understood to mean the implementation of a technology that alters a property of a surface, in particular a surface treatment can influence the composition of the substance, ie the alloying of a surface rim zone in a surface treatment, it is, as a result of the different properties of surface edge zone and core material, in particular extruder screw core, a To achieve separation of functions between the volume of an extruder screw and its surface.

A "thermochemical surface treatment" is understood to mean a surface treatment process in which a material is treated by means of a thermal-chemical action, whereby an alloying constituent is first chemically reacted and / or by a physical process, in particular by a deposition from a previously evaporated material, deposited on a base material and then diffused into this and / or reacts with constituents of the base material Thermochemi see surface treatment methods are predominantly carried out at temperatures above 500 ° C or even at temperatures above 700 ° C. Typical Verfahrenszeiträume for a thermochemi cal Obelflächenbehandlungsverfahren be up to 40 h or in some cases more than 40 h.

A "medium" is understood to mean a gaseous, a liquid or a solid substance. The "regular atmosphere" is understood as meaning the gas mixture of the earth's atmosphere. Dry air mainly consists of the two gases nitrogen (around 78.08% by volume) and oxygen (around 20.95% by volume). In addition, there are the components argon (0.93 vol .-%), carbon dioxide (0.04 vol .-%) and other gases in traces.

The prior art has heretofore provided that particularly loaded extruder screws were through-hardened and used with a heat treatment process or that bar-armored extruder screws were used.

In practice, extruder screws with an extruder screw diameter of up to approx. 50 mm were predominantly through-hardened, whereas extruder screws with a larger diameter of the extruder screw have hitherto been mainly bar-reinforced. Especially in the case of special requirements for abrasive and corrosive resistance, powder metallurgical steels are used in the prior art, which undergo a heat treatment process and are thus through-hardened. These powder metallurgical steels contain up to approx. 20% chromium as an alloying component.

Chromium forms chromium carbides with carbon, which protect against abrasive wear as hard materials.

Free chrome on the surface forms a passive layer against corrosive attack.

Depending on the heat treatment, the effect of chromium is more pronounced either as hard materials against abrasive wear or as a passive layer against corrosive attack. It has been shown that the chromium content was insufficient to fulfill both tasks, in particular in heavily loaded extruder screws, and thus the corrosive signs of wear of an extruder screw predominated.

By way of derogation, it is proposed here that first an extruder screw precursor is produced from an extruder screw alloy and then this extruder screw precursor is subjected to a thermochemical surface treatment, so that an alloy constituent diffuses into the surface edge zone of an extruder screw, whereby a differentiation of the extruder screw precursor to an extruder screw core and a surface edge zone takes place. During the thermochemical surface treatment, the extruder screw is exposed to a medium which differs from the regular earth atmosphere.

In particular, it is concretely conceivable, inter alia, that the constituent of an alloy fraction in the surface marginal zone is deliberately increased or decreased, as a result of which a property of the surface marginal zone of an extruder screw can be influenced. In particular, it is also proposed, inter alia, that an alloy constituent is deposited on the surface of an extruder screw and from there diffuses into the surface edge zone of the extruder screw.

Concretely, it is also conceivable that the extruder screw is subjected to a thermal surface treatment, with which the microstructure of the surface edge zone is influenced, so that a property of a surface edge zone can be influenced.

Advantageously, it can be achieved that a separation of functions between the surface edge zone and the extruder screw core can be achieved, wherein the surface edge zone has different material properties than the extruder screw core.

In particular, it can be achieved so advantageously that the surface edge zone has a higher surface hardness and / or a higher temperature resistance and / or a different microstructure composition and / or a lower surface friction, as a result of which the surface edge zone becomes more corrosion-resistant and / or abrasion-resistant.

Likewise, it can advantageously be achieved that the surface edge zone has a closed homogeneous material layer, as a result of which, inter alia, the coefficient of friction of the surface layer of the surface can be reduced.

Furthermore, it can be advantageously achieved that the extruder screw has better non-stick properties and can be cleaned easier and faster.

It can also be achieved in concrete terms, inter alia, advantageously that an extruder screw in the surface edge zone can have a high hardness with the advantages already mentioned above, wherein the extruder screw core can still have a higher toughness with respect to the surface layer and thus advantageously still a higher degree Deformation allows before it comes to an embrittlement break. Overall, the surface hardness can be improved without doing so as in a hardened extruder screw to fear a premature brittleness fracture or to consider in the design of the extruder screw.

In addition, can be achieved so advantageous that the product life of an extruder screw can be increased, whereby the product life costs of an extruder screw can be reduced. Furthermore, it can be advantageously achieved that starting materials can be processed with an extruder screw in the first place, which require very high corrosive and / or abrasive wear, in particular Ausgansstoffe with a very high content of fillers and / or a high proportion of flame retardants.

It can be advantageously achieved that elements from the medium diffuse into the surface edge layer, as a result of which no coating is applied, but instead of the starting material, a changed surface edge zone having a diffusion depth is achieved. This allows an extruder screw, which is hard at the same time outside and still ductile at the same time.

According to a second aspect of the invention, the object solves a method for producing an extruder screw, for use in an extruder for conveying, melting and homogenizing a starting material, in particular a polymer having starting material, wherein the extruder screw produced an extruder screw core with an extruder screw core alloy and a surface edge zone a surface edge zone alloy, the surface edge zone alloy having a chromium content of more than 20% by weight chromium, preferably more than 26% chromium by weight, more preferably more than 30% chromium by weight, with the steps (i) Preparing an extruder screw precursor from an extruder screw precursor alloy; and (ii) performing a thermochemical surface treatment to differentiate the extruder screw precursor to the extruder screw core and the surface rim zone, the extruder screw precursor exhausting a medium is protected, whereby the medium differs from a regular earth's atmosphere. According to this aspect of the invention, a method is advantageously proposed for producing an extruder screw, wherein the extruder screw is exposed to a medium deviating from a regular atmosphere during a thermochemical surface treatment, so that advantageously an extruder screw is obtained which comprises an extruder screw core with an extruder screw core alloy a surface edge zone having a surface edge zone alloy, wherein the surface edge zone alloy advantageously has a chromium content of more than 20% by weight chromium, preferably greater than 26% by weight chromium, particularly preferably greater than 30% by weight Chrome.

Preferably, the thermochemical surface treatment comprises chromating and / or boriding.

Conceptually, the following is explained:

By "chromating" or "inchromizing" is meant a thermochemical surface treatment process in which chromium diffuses at high temperatures from a medium surrounding a body to the periphery of a body, whereby the heat resistance, the hardness, the wear resistance, the corrosion resistance of the body can be increased. On the basis of the carbon content of the body to be chromed, a distinction is made between soft-ingraining and hard-chromium-plating, the processes leading to different material properties. In particular, a higher hardness but a lower maximum penetration depth of chromium is achieved in hartinchromieren.

By "boriding" is meant a thermochemical surface treatment process in which boron diffuses at high temperatures from a medium surrounding a body into the edge zone of a body, whereby the material properties of the body can be improved. Specifically, it is proposed, inter alia, that the surface boundary layer of an extruder screw is enriched with chromium by depositing chromium on the surface at temperatures between 800 ° C. and 1200 ° C., in particular at 900 ° C., and the chromium diffuses into the surface layer of the surface, In particular, the chromium is derived from a pulverulent and / or a liquid and / or a gaseous contact material.

Here, a method is proposed in which the diffused chromium forms hard chromium carbides with the existing or previously carburized carbon of the steel. Depending on the temperature and duration of treatment, differently thick carbide layers with different hardnesses are formed.

In particular, a chromium content of up to 60% can be achieved in the surface layer of the surface, and depending on the carbon content of the extruder screw precursor, additional chromium carbides can also be formed on the surface. Specifically, it is proposed, among other things, to use a hard-in-chromating process.

Advantageously, it can be achieved that a high resistance to abrasive wear is achieved by the high hardness and the extremely high temperature resistance of chromium carbide, in particular, a surface hardness of about 2,000 micro Vickers at a layer depth of 5 - 25 micrometers and a temperature resistance of 900 In particular, even a temperature resistance of 1100 ° C. can be achieved in a particularly advantageous embodiment, it also being possible to achieve a layer depth of up to 100 micrometers, regardless of the temperature resistance of a low-alloyed extruder screw precursor.

It can also be achieved with advantage that due to the specific structure of the chromium carbide layer in the surface edge zone of the extruder screw, the tendency to Adhesive wear is low, in particular a chromium carbide layer forms a particularly dense closed particularly homogeneous surface layer. Furthermore, it can be achieved particularly advantageously that chromium carbide layers have a high resistance to oxidation and corrosive media, as a result of which corrosive wear can be significantly reduced.

In addition, it can advantageously be achieved that the extruder screw core can also be hardened after the production of a chromium carbide layer in the surface edge zone, in particular during and / or after the surface treatment, so that a particularly advantageous hardness of the surface and toughness of the extruder screw core and an ideal separation of functions between the now particularly Hard surface edge zone and the ideal tough extruder screw core can be achieved.

Furthermore, it is concretely proposed, inter alia, that the surface boundary layer of an extruder screw be enriched with boron by depositing boron at temperatures between 850 ° C. and 950 ° C. on the surface and by diffusing the boron into the surface layer, in particular the boron thereby being obtained from a powdery and / or a liquid and / or a gaseous contact material.

Here, a method is proposed in which the diffused boron with the existing or previously carburized carbon of the steel forms hard boron carbides.

Depending on the temperature and duration of treatment, differently thick carbide layers with different hardnesses are formed.

It can advantageously be achieved by the high hardness and the extremely high temperature resistance of boron carbide layers, a high resistance to abrasive wear is achieved, in particular, a surface hardness of up to 2,100 Micro Vickers at a layer depth of 10 - 300 micrometers and a Temperature resistance of 500 ° C can be achieved. It can also be advantageously achieved that due to the specific structure of the boron carbide layer in the surface edge zone of the extruder screw, the tendency for adhesion wear is low, in particular a boron carbide layer forms a particularly dense, particularly homogeneous surface layer.

In addition, it can advantageously be achieved that the extruder screw core can be tempered even after the production of a boron layer in the surface edge zone, in particular during and / or after the surface treatment, so that a particularly advantageous toughness of the extruder screw core and an ideal separation of functions between the now very hard surface edge zone and the ideally tough extruder screw core can be achieved.

It is also concretely proposed, inter alia, that the surface boundary layer of an extruder screw be enriched with chromium and boron by depositing chromium and boron simultaneously or first boron then chromium or first chromium and then boron on the surface and chromium and boron in the surface layer In particular, chromium and boron originate from a pulverulent and / or a liquid and / or a gaseous contact material.

In particular, it is proposed here to use a mixed carbide layer of boron carbides and chromium carbides, while advantageously combining the respective advantages and properties with one another.

Thus, boron carbide, with its stalk-like deep structure growing to the surface, forms an ideal anchorage with the extruder screw core, while the comparatively flat chromium carbide layer advantageously permits particularly high temperature resistance and very good corrosion resistance.

Here, a method is proposed in which the diffused chromium and the diffused boron with the existing or previously carburized carbon of the steel form hard chromium carbides and hard boron carbides. Depending on the temperature and duration of treatment, different thicknesses of carbide layers with different hardnesses and properties are produced

The high hardness and the extremely high temperature resistance of the carbide layer of boron carbide and chromium carbide advantageously result in a high resistance to abrasive wear; in particular, a surface hardness of up to 2,100 microvickers can be achieved at a layer depth of 300 micrometers and a temperature resistance of 900 ° C can be achieved.

It can also be advantageously achieved that due to the specific structure of the carbide layer in the surface edge zone of the extruder screw, the tendency for adhesion wear is low, in particular a carbide layer forms a particularly dense, particularly homogeneous surface layer. In addition, it can advantageously be achieved that the extruder screw core can also be tempered after the production of such a carbide layer in the surface edge zone, in particular during and / or after the surface treatment, so that a particularly advantageous toughness of the extruder screw core and an ideal separation of functions between the now very hard surface edge zone and the ideal tough extruder screw core can be achieved.

Furthermore, it can be achieved particularly advantageously that chromium carbide layers have a high resistance to oxidation and corrosive media, as a result of which corrosive wear can be significantly reduced.

In a particularly preferred embodiment, the thermochemical surface treatment comprises a chromating and / or boriding after carburizing.

Conceptually, the following is explained: By "carburizing" is meant a heat treatment process in which carbon is diffused into metals at high temperatures by heating the body to be carburized in a carbon-containing medium to high temperatures.The carburizing is not or only poorly due to their low carbon content In most cases, only the surface layer is enriched with carbon so that more martensite is formed there than in the core and a hard surface layer is formed, whereby the core can usually remain tough and soft A large number of metal carbides, such as tungsten carbide, chromium carbide, boron carbide or tantalum carbide, can be produced in a targeted manner.

In particular, it is proposed to correspondingly carburize an extruder screw precursor prior to the surface treatment and thus to advantageously increase the carbon content before it is subjected to a surface treatment process envisaged here, so that further combinations of desired material properties can be advantageously achieved, in particular the functional separation between the extruder screw core region and the surface edge area even better advantageous. Thus, it can advantageously be achieved that the extruder has a screw core area, a high toughness, while the surface edge area is particularly resistant to carbides, in particular by chromium carbides and / or boron carbides. Inter alia, it is proposed to carburize the extruder screw precursor such that a carbon content of at least 0.2% by weight is achieved in the surface rim zone, preferably a carbon content of at least 0.6% by weight, more preferably a carbon content of more than 0.6 Wt .-% up to 2.2 wt .-%. In individual cases, it is also proposed that a carbon content of more than 2.2% by weight be set in the surface rim zone.

Advantageously, this can be achieved by the extruder screw precursor before the thermochemi see surface treatment method reaches a higher carbon content, which can see more carbides in the surface edge zone can be achieved with the thermochemi see surface treatment method. In particular, an alloy can be achieved which can be hard-ingrained, whereby particularly advantageous material properties can be achieved.

It should be expressly understood that the foregoing values for the carbon content should not be construed as being in any way limited, but rather should be exceeded or disregarded on an engineering scale without departing from the described aspect of the invention. In simple words, the values are intended to provide an indication of the size of the range of carbon content proposed here.

Preferably, the thermochemical surface treatment has a temperature above 700 ° C, preferably a temperature above 750 ° C, more preferably a temperature above 800 ° C. Advantageously, the most effective possible diffusion of chromium and / or boron can thus be achieved.

It should be expressly understood that the foregoing values for the temperature are not to be construed as being in any way limited, but rather are to be exceeded or disregarded on an engineering scale without departing from the described aspect of the invention. In simple terms, the values are meant to indicate the size of the minimum temperature proposed here for the thermochemical surface treatment method.

In a particularly preferred embodiment, the extruder screw is hardened after the thermochemical surface treatment. Conceptually, the following is explained:

A "heat treatment process" is a process or combination of several processes for treating a metallic workpiece, which is heated and cooled in a particular process to alter material properties of the carbon or nitrogen content, or crystal lattice or crystal structure. Examples of heat treatment processes are hardening and tempering.

"Tempering" describes a combined heat treatment of metals, consisting of hardening and subsequent tempering.This heat treatment is intended to put the material in a state of high toughness properties with high tensile strength and hardness, respectively.The best yield ratio and the highest toughness are achieved during tempering If the material is to be tempered, it is preferable to use tempering as soon as possible after hardening in order to avoid any cracking caused by internal stresses The ability of a material to form a stable martensite structure under certain conditions requires a carbon content of at least 0.2% to 0.3% of the steel for classic tempering also referred to as tempered steel (usually 0.35% to 0.6% carbon). "Hardening" is the process of rapidly cooling out the austenite phase, and it is important that the material be fully austenitized before, ie the ferrite must be complete and the carbides must be almost completely dissolved, and after austenitizing, the material is quenched. As a rule, a martensitic microstructure, in some materials also an intermediate or a mixture of martensite and intermediate grade, is produced, which has the highest possible hardness.

"Quenching" refers to the rapid cooling of the heated workpiece by the use of quenching agents, preferably water, oil or air, in which the choice of quenching agent influences the quenching rate to be achieved and thus the resulting structure. The main differences between tempering and tempering can be found in the tempering process step "annealing" means reheating the quenched steel After quenching, an immediate annealing step at about 150 ° C is beneficial The resulting brittle tetragonal martensite (coniferous martensite) is converted into the cubic martensite with the excretion of fine carbides, which has a smaller volume, thus relaxing the grain lattice and thus eliminating the "glass hardness" of the material In addition, existing residual austenite is further decomposed by diffusion processes and converted into martensite with a further increase in hardness.The process of tempering can take place in several time-shifting steps from stress cracks to minimie To avoid or avoid a starting as soon as possible after curing is desirable. The aim of the tempering is the reduction of material stresses caused by hardening and the final setting of the desired technological properties such as tensile strength, yield strength, elongation and constriction. In the structure, several processes take place during the start. During tempering no crystalline change takes place. The hardness is reduced by the onset Martensitzerfall with the formation of fine, ε-carbides (above about 100 ° C) and the excretion of fine carbides "Fe3C" (cementite, above about 250 ° C). The degree of carbide precipitation and its size increase with the height of the tempering temperature and thus the hardness decreases to the same extent. The structure consists of a complete tempering of ferritic matrix with embedded carbides. At about 200 ° C, tetragonal martensite converts to cubic martensite.

"Relaxing", often referred to as "stress-free annealing", serves to reduce stresses in the material. A structural transformation does not take place. Depending on the temperature and holding time, a slight shaping effect is found in the microstructure, and the strength can easily fall off. "Austenite" is the metallographic term for the face-centered cubic modification (phase) of pure iron and its mixed crystals, and if the material contains carbon, austenite is a mixed-crystal intercalation crystal.

"Martensite" is a metastable structure in metals that is produced diffusion-free and athermic by a cooperative shear motion from the initial structure, where the material must be cooled from the temperature of a high-temperature phase (austenite) below the equilibrium temperature to a low-temperature phase (ferrite), in particular The subcooling below the equilibrium temperature must be deep enough to provide the necessary driving force for the athermal phase transformation, but must also be fast enough to prevent diffusion.

"Ferrite" is the metallographic term for the cubic-body-centered modification (phase) of pure iron and its mixed crystals, which specifically proposes, inter alia, hardening the extruder screw during the surface treatment or after the surface treatment with a low tempering temperature This advantageously allows an ideal separation of functions between a hard surface edge layer and a hard extruder screw core, wherein the hardness of the extruder screw core, depending on the alloy of the extruder screw precursor, the tempering temperature and tempering process, harder compared to the hardness of the surface layer surface, softer or, in particular, equally hard.This additional heat treatment process allows the mechanical properties of the extruder screw to be particularly advantageous for the desired purpose be set.

In a particularly advantageous embodiment, it is further proposed here to harden the extruder screw during the surface treatment or after the surface treatment with a higher tempering temperature, thereby advantageously taking place in the extruder. The lower core can be achieved a lower hardness compared to the surface layer of the surface. This advantageously creates an ideal separation of functions between a hard surface edge layer and a tough extruder screw core. By means of this additional heat treatment process, the mechanical properties of the extruder screw can be set particularly advantageously to the desired target values. It should be expressly understood that the subject matter of the second aspect may be advantageously combined with the subject matter of the foregoing aspect of the invention, cumulatively either singly or in any combination.

According to a third aspect of the invention, the object is achieved by an extruder screw for use in an extruder for conveying, melting and homogenizing a starting material, in particular a starting material comprising a polymer, wherein the extruder screw has an extruder screw core with an extruder screw core alloy and a surface rim zone with an extruder screw Surface edge zone alloy, wherein the surface edge zone has a surface treatment, in particular an extruder screw, which has been produced by a method according to the first and / or second aspect of the invention, wherein the surface edge zone alloy of a treated surface compared to the extruder screw core alloy has an increased chromium content and / or an increased boron content having.

Conceptually, the following is explained:

By "% by weight" is meant the weight percent of an alloying ingredient in an alloy, especially an extruder screw core alloy or a surface fringe zone alloy.

The prior art has heretofore provided that particularly loaded extruder screws were through-hardened and used with a heat treatment process or that bar-armored extruder screws were used. By way of derogation, an extruder screw is proposed which has an alloy in its surface edge zone which has an increased chromium content and / or an increased proportion of boron compared with the extruder screw core alloy.

In this way, an alternative can be made available to the prior art which has particularly advantageous material properties, in particular advantageous wear resistance, advantageous corrosion resistance, advantageous heat resistance and / or advantageous surface hardness, wherein the extruder screw core can furthermore have an advantageously high toughness.

In a particularly preferred embodiment, the Oberflächenrandzonenlegie- tion has a chromium content of more than 20 wt .-% chromium, preferably of more than 26 wt .-% chromium, more preferably of more than 30 wt .-% chromium.

Advantageously, as a result, particularly advantageous material characteristics can be achieved.

It should be expressly understood that the foregoing values for the chromium content are not to be construed as being in any way limited, but rather are to be exceeded or disregarded on an engineering scale without departing from the described aspect of the invention. In simple words, the values are intended to provide an indication of the size of the chromium portion proposed here.

Optionally, the surface edge zone has a layer thickness in a range from 5 μm to 50 μm, preferably a layer thickness in a range from 8 μm to 35 μm, particularly preferably a layer thickness in a range from 10 μm to 20 μm. Advantageously, it can be achieved that a particularly robust surface edge zone can be achieved with a high impact resistance, while at the same time the vast proportion of material of the extruder screw can have a tough material behavior. Thus, a consensus from a wear protection of the extruder screw and the robustness of the wear protection layer of the extruder screw is advantageously achieved. In particular, it can be achieved so advantageously that the wear protection layer does not prematurely partially or completely embrittled and / or cracked by an occurring load, and flakes off prematurely as a result.

It should be expressly pointed out that the above values for the layer thickness of the surface edge zone should not be understood as sharp boundaries, but rather should be exceeded or fallen below on an engineering scale, without departing from the described aspect of the invention. In simple words, the values are intended to provide an indication of the size of the surface boundary layer thickness range proposed here.

In a particularly advantageous embodiment, the surface treatment comprises a region which lies on a gear surface.

Conceptually, the following is explained:

A "region" is understood to mean a spatially delimitable extension, in particular a spatially delimitable extension of the surface edge zone of an extruder screw. A "transition surface" is understood to be the surface of a screw flight of an extruder screw. An extruder screw has one or more screw flights, in which material flow is conveyed from the screw inlet to the screw outlet.

It is concretely conceivable, inter alia, that a surface treatment extends only to the desired area, ie the area in which the desired change in the properties can be advantageously used.

In particular, it is specifically proposed, inter alia, that areas of the surface of an extruder screw should not be subjected to a surface treatment which does not belong to the surface of the gear, in particular end-side forming and mating surfaces of the extruder screw. Advantageously, it can thus be achieved that the properties caused by means of a surface treatment and desired properties extend only to the region in which they are also desired.

In particular, it can be achieved that for some areas undesired properties, which are caused by a surface treatment method, do not extend to those areas in which they are predominantly undesirable, in particular no changes in shape in a region of frontal shaping and fitting surfaces of a Extruder screw be caused. These can be advantageously avoided by this aspect of the invention. Preferably, the surface treatment is effected by a thermochemical method.

Specifically, it is proposed here that in the surface edge layer of an extruder screw, a particularly effective protection against wear is achieved by an application of a thermochemical surface treatment method. Advantageously, it can thus be achieved that specific properties of a surface edge layer of an extruder screw can be achieved particularly advantageously with a thermochemical surface treatment method.

Particularly preferably, the extruder screw core alloy has a carbon content of at least 0.2 wt .-%, preferably a carbon content of at least 0.6 wt .-%, more preferably a carbon content of more than 0.6 wt .-% to 2.2 wt .-%.

It is also possible in particular that the extruder screw core alloy has a carbon content of up to 2.2% by weight and in some cases more than 2.2% by weight. In concrete terms, it is proposed, inter alia, that the carbon content of an extruder screw pellet alloy is at least 0.2% by weight, particularly preferably a carbon content of at least 0.6% by weight. Due to the high carbon content, the potential of a surface treatment process can be better exploited, so that better material properties can advantageously be achieved thereby. It should be expressly understood that the above values for the carbon content should not be construed as being in any way limited, but rather are to be exceeded or fallen short of on an engineering scale without departing from the described aspect of the invention. In simple words, the values are intended to provide an indication of the size of the range of carbon content proposed here. In a particularly preferred embodiment, the extruder screw is hardened.

By this additional heat treatment process so the mechanical properties of the extruder screw can be set even better advantageous.

In particular, advantageously a high hardness and / or a high wear resistance and / or a high heat resistance and / or a corrosion resistance in the surface edge zone and within the framework of an ideal separation of functions between the surface edge zone and the extruder screw core an advantageously tough extruder screw core can be achieved.

It should be expressly understood that the subject matter of the third aspect may be advantageously combined with the subject matter of the foregoing aspects of the invention, cumulatively, either singly or in any combination.

According to a fourth aspect of the invention, the object is achieved by an extruder with an extruder screw according to the third aspect of the invention and / or an extruder with an extruder screw, which has been produced by a method according to the first and / or second aspect of the invention. It is understood that the advantages of an extruder screw according to the third aspect of the invention as described above and / or the advantages of an extruder screw produced by a method according to the first and / or second aspect of the invention are as described above , immediately to an extruder with an extruder screw according to the third aspect of the invention and / or with an extruder screw, which has been produced by a method according to the first and / or second aspect of the invention extend.

It should be expressly understood that the subject matter of the fourth aspect may be advantageously combined with the subject matter of the foregoing aspects of the invention, cumulatively either alone or in any combination. The invention will be explained in more detail with reference to an embodiment with reference to the drawing. Show there

Fig. 1 shows schematically temperature-time courses for the heat treatment process

 Hardening and tempering, and

2 schematically shows stress-strain curves for a hardened, a tempered and a normalized material.

The temperature-time courses of heat treatment processes in FIG. 1 consist of one or more courses 20, 21 of the temperature 12 of a heat treatment process over time 13.

Heat treatment processes are divided over their course of time 13 into the process phases austenitizing 14, quenching 15 and tempering 16.

The temporal transition between the austenitizing 14 and quenching 15 process phases occurs almost seamlessly at the process boundary 17. Fast quasi-seamless quenching is important in keeping the carbon in the lattice structure of the material constrained and non-diffused. The temporal transition between the phases of quenching 15 and tempering 16 can take place with a time delay. The process boundary 18, which runs between the quenching process phase 15 and the process phase tempering 16, is therefore not necessarily passed through continuously. Rather, it is common for additional time to pass between quenching 15 and tempering 16 (not indicated in the schematic illustration in FIG. 2).

The temperature-time courses 11 are dependent on the material, the selected heat treatment process and the dimensions of the material. These dependencies are disregarded in this schematic consideration. However, the process phases austenitizing 14 and quenching 15 are independent of the process phase tempering 16.

When tempering 16, the main differences in the temperature-time courses are 11. Considered are the gradients for the heat treatment process hardening 20 and the heat treatment process annealing 21. The temperature reached during tempering 21 12 is significantly higher than during curing 20. Here also occur Dependent on the material, the method and the dimensions of the component. Equally, in all conceivable combinations, however, the significantly higher temperature 12 remains during tempering 21 compared with hardening 20.

The different heat treatment methods lead to different material behavior, in particular to different stress-strain curves 31. The stress-strain curves 31 in FIG. 2 consist of a stress 32, an elongation 33 and the progressions 34, 35, 36 of a material heat-treated differently ( here exemplary C45).

A hardened material is very strong and reaches a course under load 34. It achieves a very high internal stress 32 before the break due to overloading but only a small elongation 33. It can withstand particularly high loads and is not suitable for absorbing plastic deformations. He is rather brittle.

A tempered material is less strong than a hardened steel but much tougher than a hardened steel. It reaches under load a curve 35 and before the break because of overloading a high internal stress 32 at a high degree of elongation 33. It is compared to the cured material less solid, but can accommodate higher plastic strains to break.

A normally annealed material has in its course 36 to break due to overloading in comparison, the lowest internal stress 32 at the highest level of plastic extensibility 33.

List of reference numbers used

11 temperature-time profiles

12 temperature

 13 time

 14 Austenitizing

 15 quenching

 16 starting

 17 process limit

 18 process limit

 20 hardening

 21 Reward

 31 stress-strain curves

32 voltage

 33 stretching

 34 Hardened material

 35 tempered material

 36 Normalized material

Claims

claims: A process for producing an extruder screw, for use in an extruder for conveying, melting and homogenizing a starting material, in particular a starting material comprising a polymer, comprising the steps (i) preparing an extruder screw precursor from an extruder screw precursor solution; (ii) performing a thermochemical surface treatment to differentiate the extruder screw precursor to an extruder screw core and a surface rim zone, wherein the extruder screw precursor is exposed to a medium, the medium being different from a regular earth atmosphere by an increased chromium and / or boron content. 2. A process for producing an extruder screw, for use in an extruder for conveying, melting and homogenizing a starting material, in particular a starting material comprising a polymer, the extruder screw produced being an extruder screw core with an extruder screw core alloy and a surface edge zone having a surface edge zone alloy wherein the surface erosion zone has a chromium content of more than 20% by weight chromium, preferably more than 26% chromium by weight, more preferably more than 30% chromium by weight, with the steps (i) preparing an extruder screw precursor from an extruder screw precursor alloy; (ii) performing a thermochemical surface treatment to differentiate the extruder screw precursor to the extruder screw core and the surface rim zone, wherein the extruder screw precursor is exposed to a medium, the medium being different from a regular earth atmosphere. 3. The method according to any one of claims 1 or 2, characterized in that the thermochemical surface treatment comprises a chromating and / or boriding. 4. The method according to any one of the preceding claims, characterized in that the thermochemical surface treatment comprises a chromating and / or boriding after carburizing. 5. The method according to any one of the preceding claims, characterized in that the thermochemical surface treatment has a temperature above 700 ° C, preferably a temperature above 750 ° C, more preferably a temperature above 800 ° C. 6. The method according to any one of the preceding claims, characterized in that the extruder screw is hardened after thermochemi see surface treatment. An extruder screw for use in an extruder for conveying, melting and homogenizing a starting material, in particular a polymer having starting material, the extruder screw having an extruder screw core with an extruder screw core alloy and a surface edge zone with a surface edge zone alloy, wherein the surface edge zone has a surface treatment, in particular extruder screw, which has been produced by a method according to one of claims 1 to 6, characterized in that the surface edge zone alloy of a treated surface has an increased chromium content and / or increased boron content relative to the extruder screw core alloy. 8. extruder screw according to claim 7, characterized in that the Oberflächenrandzonenlegierung has a chromium content of more than 20 wt .-% chromium, preferably of more than 26 wt .-% chromium, more preferably of more than 30 wt .-% chromium. 9. extruder screw according to one of claims 7 or 8, characterized in that the surface edge zone has a layer thickness in a range of 5 μιη to 50 μιη, preferably a layer thickness in a range of 8 μιη to 35 μιη, particularly preferably a layer thickness in a range from 10 μιη to 20 μιη. 10. extruder screw according to one of claims 7 to 9, characterized in that the surface treatment comprises an area which is located on a Gangober- surface. 11. extruder screw according to one of claims 7 to 10, characterized in that the surface treatment is effected with a thermochemi see method. 12. Extruder screw according to one of claims 7 to 11, characterized in that the extruder screw core alloy has a carbon content of at least 0.2 wt .-%, preferably a carbon content of at least 0.6 wt .-%, more preferably a carbon content of more than 0.6 wt .-% up to 2.2 wt .-%. 13. extruder screw according to one of claims 7 to 12, characterized in that the extruder screw is cured. 14. Extruder with an extruder screw according to one of claims 7 to 1 and / or extruder with an extruder screw, which has been produced by a method according to any one of claims 1 to 6. 15. Plastics forming plant, comprising an extruder according to claim 14, a plastic feed to the extruder, a continuation to a nozzle and / or an injector and / or a granule producer and / or a nonwoven producer and / or a Folienbahnerzeuger. 16. A plastic molding process using a plant according to claim 15, comprising the steps a. Extruding a plastic with an extruder according to claim 14 to produce an extrudate; b. Shaping the extrudate. c.